A variety of DNA hybridization techniques are available for detecting the presence of one or more selected polynucleotide sequences in a sample containing a large number of sequence regions. In a simple method, which relies on fragment capture and labeling, a fragment containing a selected sequence is captured by hybridization to an immobilized probe. The captured fragment can be labeled by hybridization to a second probe which contains a detectable reporter moiety.
Another widely used method is Southern blotting. In this method, a mixture of DNA fragments in a sample are fractionated by gel electrophoresis, then fixed on a nitrocellulose filter. By reacting the filter with one or more labeled probes under hybridization conditions, the presence of bands containing the probe sequence can be identified. The method is especially useful for identifying fragments in a restriction-enzyme DNA digest which contain a given probe sequence, and for analyzing restriction-fragment length polymorphisms (RFLPs).
Another approach to detecting the presence of a given sequence or sequences in a polynucleotide sample involves selective amplification of the sequence(s) by polymerase chain reaction (Mullis, Saiki). In this method, primers complementary to opposite end portions of the selected sequence(s) are used to promote, in conjunction with thermal cycling, successive rounds of primer-initiated replication. The amplified sequence may be readily identified by a variety of techniques. This approach is particularly useful for detecting the presence of low-copy sequences in a polynucleotide-containing sample, e.g., for detecting pathogen sequences in a body-fluid sample.
More recently, methods of identifying known target sequences by probe ligation methods have been reported (Wu, Whiteley, Lundegren, Winn-Deen). In one approach, known as oligonucleotide ligation assay (OLA), two probes or probe elements which span a target region of interest are hybridized with the target region. Where the probe elements match (basepair with) adjacent target bases at the confronting ends of the probe elements, the two elements can be joined by ligation, e.g., by treatment with ligase. The ligated probe element is then assayed, evidencing the presence of the target sequence.
In a modification of this approach, the ligated probe elements act as a template for a pair of complementary probe elements. With continued cycles of denaturation, reannealing and ligation in the presence of the two complementary pairs of probe elements, the target sequence is amplified geometrically, allowing very small amounts of target sequence to be detected and/or amplified. This approach is also referred to as Ligase Chain Reaction (LCR).
There is a growing need, e.g., in the field of genetic screening, for methods useful in detecting the presence or absence of each of a large number of sequences in a target polynucleotide. For example, as many as 150 different mutations have been associated with cystic fibrosis. In screening for genetic predisposition to this disease, it is optimal to test all of the possible different gene sequence mutations in the subject's genomic DNA, in order to make a positive identification of a "cystic fibrosis". Ideally, one would like to test for the presence or absence of all of the possible mutation sites in a single assay.
These prior-art methods described above are not readily adaptable for use in detecting multiple selected sequences in a convenient, automated single-assay format. It is therefore desirable to provide a rapid, single-assay format for detecting the presence or absence of multiple selected sequences in a polynucleotide sample.
In another technical area, attempts to increase the throughput and degree of automation in DNA sequencing has lead to the development of sequencing approaches making use of capillary electrophoresis, e.g. Huang et al, Anal. Chem., Vol. 64, pgs. 2149-2154 (1992); Swerdlow et al, Nucleic Acids Research, Vol. 18, pgs. 1415-1419 (1990), and the like. A major impediment to the widespread application of these approaches is the requirement that gels be used in the separation medium of the capillaries. It is still very time consuming and difficult to load and/or form gels in the capillaries that have sufficiently uniform quality for reliable and convenient sequencing. High speed DNA sequencing for either diagnostic applications or for genomic analysis would make a major advance if means were available that permitted differently sized polynucleotides to be separated electrophoretically in a non-sieving liquid medium which could be readily loaded into capillaries without the time and quality control problems associated with gel separation media.